PROJECT SUMMARY Polymicrobial interactions, or coaggregation, are critical in the development of oral biofilms termed dental plaque. Coaggregation of Actinomyces oris ? a key colonizer of the oral biofilms ? with early colonizers is mediated by type 2 fimbriae, which consist of the fimbrial shaft FimA and tip protein FimB. The assembly of type 2 fimbriae on the bacteria surface requires a conserved transpeptidase enzyme called sortase. Although fimbria-mediated coaggregation in A. oris has been known for more than 3 decades, the molecular entity of the fimbrial adhesin(s) required for this process is only recently revealed. It was shown that the coaggregation factor CafA, not genetically linked to the type 2 fimbrial gene cluster, is located at the fimbrial tip, forming a distinct fimbrial structure, and that deletion of cafA abrogates A. oris coaggregation with the early colonizer Streptococcus oralis. How CafA is incorporated into the type 2 fimbriae and how it mediates bacterial coaggregation are completely unknown. Preliminary studies reveal that CafA and FimB harbor a twin-arginine translocon (Tat) signal peptide, unlike FimA, and that CafA shares with FimB a sortase-recognition cell wall sorting signal (CWSS) with several conserved motifs, which are absent in other sortase-mediated cell wall anchored proteins. Furthermore, while the wild-type strain A. oris MG1 adheres to human gingival fibroblasts, which is enhanced by sialidase treatment, an A. oris cafA deletion mutant is defective in this process, suggesting that CafA recognizes a polysaccharide receptor that is similar to that of oral streptococci. Based on these novel findings, we hypothesize that by molecular mimicry Tat-transported CafA hijacks the sortase pilus assembly machine to be displayed on the pilus tip for its adhesive function. We will examine this hypothesis by two specific aims. By mutational analysis of the CWSS and genetic disruption of the Tat translocon, we will elucidate the mechanism of pilus hijacking by Tat-transported CafA. Secondly, by a combination of mutational analysis, coaggregation assay, and genomic sequencing of differentially co-aggregating clinical isolates, we aim to elucidate the mechanism of CafA-mediated polymicrobial and inter-kingdom interactions.